Casting process employing soluble cores



April 4, 1967 M. E. TOWNSEND ET AL Filed May 24, 1965 CASTING PROCESSEMPLOYING SOLUBLE CORES 2 Sheets-Sheet l SALT FURNACE 585g STORAGEMATERIAL DIE CASTING PRODUCT APPARATUS WIZH CORE CORE SEP RATOR Inn.REMOVAL A r v 34 4o] PRo0ucr WATfR FIG. I j

INVENTORS l MERLYN E. TOWNSEND FIG. 2

C/ A TTORNE;

April 4, 1967 TOWNSEND ET AL 3,311,956

' CASTING PROCESS EMPLOYING SOLUBLE CORES Filed May 24, 1965 2Sheets-Sheet 2 FIG.5

INVENTORS hgERLYN amwygamo ue E a. GUE BY Do /$1.0 6. LA RuE ATTORNEYUnited States Patent 3,311,956 CASTING PROCESS EMPLOYING SOLUBLE CORESMerlyn E. Townsend, Concord, Eugene J. Guelda, Oakland, and Donald G. LaRue, Orinda, Califi, assignors to Kaiser Aluminum & ChemicalCorporation, Oakland, Calif., a corporation of Delaware Filed May 24,1965, Ser. No. 458,066 3 Claims. (Cl. 22-194) This invention relates toimprovements in the casting of materials, such as metals. Moreparticularly, it is concerned with providing improved disposable orreadily removable cores for use in casting from such materials articles,portions of which can have hollow configurations, hereinafter calledhollow articles, and with methods of making or preparing these cores forsuch use. This invention also relates to novel methods of casting whileutilizing these improved cores.

Cores for use in casting or molding of hollow metal articles and thelike such as cores of sand, plaster or other materials which have thecharacteristic of being easily disintegrated in order to facilitatetheir removal from final solidified castings have been and are currentlybeing used, Although cores of this type can be utilized in the practiceof sand casting and semi-permanent mold casting with some measure ofsuccess, these cores ordinarily lack the requisite strength to withstandthe destructive forces to which molding materials are subjected inpresent day die casting practices. Metallic cores are also availableand, While these metallic cores may possess the requiste strength toWithstand the present day high molding pressures of die casting,problems are involved in readily removing these cores from the finishedarticle. As a consequence, metallic cores are ordinarily limited intheir usage to cores of simple configurations that have relatively fewor no undercuts or backdrafts and which can be readily removed ascomplete units from the finished castmg.

Disposable or removable cores made of water-soluble salts, such assodium chloride, have been previously proposed for use in castingpractices and these cores can take relatively complex shapes orconfigurations. Although in instances where a particular castingapplication requires low molding pressures, these particular salt corescan be used; in the case of high pressure molding processes, however,such as die casting of metals, wherein a given core which may also be ofa rather complex shape must be capable of withstanding high compressiveforces throughout, these particular salt cores cannot be used because,among other considerations, they lack the strength to withstand theunbalanced forces to which they are subjected. The term core as usedherein is meant to encompass either an insert in a mold that shapes theinterior of an article being cast therein or an insert in a mold thatshapes the exterior of the article being cast therein or variouscombinations of both. In other words, the term core simply means a moldinsert used in the formation of either external or internal surfaceportions of an article.

Accordingly, the instant invention is concerned with providing a uniquedisposable core which has the requisite strength properties to withstandthe high compressive forces which are experienced in present day diecasting practices together with a unique method for making thesedisposable cores whereby these cores, even when provided with anintricate or complex configuration, will still be able to withstand thehigh casting or molding pressures to which they are subjected. Theinstant invention is also concerned with providing a novel method ofcasting while employing the unique cores of the instant invention.

The material of the cores of the invention possesses certainadvantageous physical properties hereinafter discussed in detail whichenhance its ability to be fused and cast into cores of complex andirregular configurations and shapes.

These and other purposes and advantages of the instant invention willbecome more apparent from a review of the ensuing detailed descriptiontaken in conjunction with the appended drawings wherein:

FIGURE 1 is a schematic diagram showing the interrelation of the severalprocessing steps employed during the manufacturing of a hollow die castarticle and illustrates a preferred series of process steps forpreforming the disposable core element in conjunction with a series ofprocess steps for forming an improved die cast article or the like byuse of the aforesaid preformed core element;

FIGURE 2 is a perspective view of a representative disposable coreelement that can be produced by the series of manipulative steps setforth in FIG. 1;

FIGURE 3 is a perspective view of an improved hollow articlemanufactured by use of certain of the manipulative steps illustrated inFIG. 1

FIGURE 4 is a sectional view generally taken along line 4-4 of FIG. 3and discloses the disposition of the core element in the hollow interiorof the article being produced prior to dissolution and removal of thecore element therefrom; and

FIGURE 5 is a perspective view of a further design for a disposable coreelement which can be formed by certain of the process steps illustratedin FIG. 1.

In the casting art, there has been a real need for a disposable orreadily removable core which does not possess the serious shortcomingsand handicaps of prior art cores; i.e., a disposable core which amongother things has minimal shrinkage characteristics, can be easilyremoved from the finished casting and yet have sufficient strength towithstand the conditions to which it would be subjected in present daymolding operations. The essential properties required in such an idealdisposable core may be summarized as follows:

(1) The core material should have minimal shrinkage as it passes from aliquid to a solid state or condition.

(2) The core should possess sufiicient strength to withstand all normalconditions to which it is subjected in the casting operation.

(3) The core should be capable of being formed so as to be an accuratereproduction of the core mold cavity and have good surface definitionand dimensional accuracy so that even very complex castings can be madeWhile using the same.

(4) The core should be capable of being initially solidified from itsmolten condition in molds made of any one of a number of conventionalmold materials, such as sand, metal, plaster, etc.

(5) The core material after use should be readily recoverable by simpleeconomical procedures and be reusable indefinitely, in other words, thecore material should be readily dissolvable in and separable from andunreactive with water or an aqueous solution.

(6) The core should have a melting point which is higher than themelting point of the material which is to be finally cast or moldedaround the core so as not to melt, abrade, or distort during the castingor molding operation.

(7) The core should have a higher coefiicient of thermal expansion thanthe material being cast around it thereby eliminating the inducement ofdeleterious stresses in the cast material as it and the heated corecontract during the cooling of the casting.

(8) The core, when solidified, should present a substantially smoothnon-porous surface or surfaces to the material being cast therearoundwhereby the final cast product will likewise have a correspondinglysmooth surface or surfaces which require little if any, machining orgrinding.

(9) The core should have relatively low thermal conductivity so as notto extract heat too rapidly from the molten material being cast aroundit.

(10) The core should be unreactive with the material being cast aroundit.

(11) The core should not undergo reaction or decomposition under normalconditions of use.

(12) The core material should be essentially nontoxic so as to becapable of use with a minimum amount of danger to the user.

(13) The core material should be relatively inexpensive and easilyobtainable.

(14) Finally, the molten core material should have good pouringproperties including fluidity so as to completely fill the core moldcavity.

It has been found that anhydrous calcium chloride meets the above severerequirements and exhibits all or substantially all of the propertieshereinabove set forth. Of particular significance is the fact thatanhydrous calcium chloride is characterized by having a cubicalshrinkage of less than 1% as it solidifies from the molten to solidstate. Within limits the anhydrous calcium chloride can be used inadmixture with other materials without adversely affecting its minimalshrinkage characteristics. For example, it has been found that up to 10%by weight of potassium chloride can be mixed with the anhydrous calciumchloride and the resultant molten mixture will shrink less than as itsolidifies.

In this connection, it is to be understood of course, that, although theamount of shrinkage that can be tolerated for any given article to beproduced by use of the cores of the instant invention can vary over asubstantial range, the use of anhydrous calcium chloride is stilladvantageous because of its above indicated significant minimalshrinkage characteristic. It is to be understood that the invention isnot limited to C.P. (chemically pure) or U.S.P. (United StatesPharmacopoeia) grades of calcium chloride. Small quantities ofadditives, impurities, and fillers can be combined with the anhydrouscalcium chloride, either in solution or as a dispersion, so long astheir presence does not adversely affect the ability of the anhydrouscalicum chloride to meet the above outlined criteria. Examples of suchimpurities, additives, and fillers are sand or alumina which are ofsufiiciently fine particle size and/or in a sufficiently small quantityso as to form a uniform dispersion in the core materials and at the sametime not adversely affect the ability of the anhydrous calcium chlorideto meet the above-outlined criteria. In this connection, it has beenfound that the amount and type of impurities normally present incommercially available calcium chloride (e.g., about 94-97% pure) do notadversely affect the ability of the anhydrous calcium chloride to meetthe above-outlined criteria. Commercially available calciumchloridenormally contains up to 5% other alkali chlorides, such assodium chloride. Thus, it is to be understood that, as used herein, theterms consisting essentially of means the calicum chloride may includeother ingredients which do not materially affect the basic and novelcharacteristics of the core elements of the invention.

7 The anhydrous calcium chloride core elements of the instant inventionare normally self-supporting but may have metal pins or the likeincorporated therein to support and position the core elements in thedie casting apparatus. The core elements can be cast around the pins asthe cores are being formed of the anhydrous calcium chloride. Removal ofthe pins from the core element after the casting operation mayadvantageously expose more surface area of the core element to waterthan would otherwise be the case thereby resulting in a more rapiddissolution of the core. If desired, the pins may be left in placewithin the die casting and the core element dissolved from around them.Metal inserts of various configurations may be positioned in the diecasting in the same manner.

Ordinarily, calcium chloride is a deliquescent material and readilyabsorbs water from its surroundings to form hydrous calcium chloride.When fully hydrated it has the chemical formula CaCl -GH O. Hydratedsalts are inherently unstable and when heated liberate their water ofhydration. Under the high temperature-high pressure conditions presentin the die casting of metal, the liberated water from a core made of ahydrated salt will ordinarily be converted to steam under pressure whichwill force its way through the metal leaving blow holes therein as wellas causing a serious safety problem during casting operations.

When die casting a metal, such as aluminum or aluminum alloys around acore of hydrous calcium chloride, an additional problem is normallypresented. The water liberated as the salt is heated reacts with thealuminum or aluminum alloy to form aluminum oxide and to liberatehydrogen gas. Inasmuch as molten aluminum has a substantial affinity forhydrogen, the metal under some circumstances could tend to become gassyand castings of high porosity could result.

As mentioned above, hydrated salts are also inherently thermallyunstable. On the other hand, because of the criteria listed above, thecore material must be a thermally stable compound. Even though calciumchloride is deliquescent in nature, it has been found that under certain critical operating conditions, both in the core formation and inthe casting operation, anhydrous calcium chloride is an excellent corematerial meeting substantially all of the above listed criteria.

In accordance with the present invention, a disposable core element isprovided for casting consisting essentially of a fused cast body ofanhydrous calcium chloride. As used herein, the term fuse means: blend,integrate, or to :blend by melting together. The present invention isalso concerned with providing a process for making the disposable coreelement and involves the preferred steps of heating a preselected amountof calcium chloride to a temperature sufficient to convert it and tomaintain it in the molten state and to ensure the removal of all waterof hydration whereby a fused molten body of anhydrous calcium chlorideis obtained. The anhydrous molten calcium chloride i then cast into thecavity of a suitable mold. This mold may be a heated mold which is at asecond temperature selectively lower than the first mentionedtemperature, for example, about 500 F., in the case of a metal mold. Thecavity of the mold has an overall configuration corresponding to thedesired final shape of the core element being fabricated. The coreelement is removed from the core mold as soon as solidification of thesame has been substantially completed.

In order to retain the anhydrous condition of the core element, theinstant invention contemplates that after removal of the core elementfrom the core forming mold, it will be transferred to a suitabledepository within which it can be maintained in a true anhydrouscondition until needed in a casting operation. Such a depository canconsist of a warming furnace wherein the core can be kept at a minimumtemperature of 200 C. or 392 F. (the temperature at which hydrou calciumchloride loses all of its water of hydration) or the depository cancomprise a sealed container purged of all water vapor. Withconfigurations where the core element is restricted in its contractionwithin the mold, it is important that the fused cast core element beremoved from the core mold after solidification temperature has beenreached and as soon as solidification of the core is substantiallycomplete. If the fused cast core element were permitted to remain in thecore mold and to further cool therein after solidification, in the caseof certain configurations, undesirable: stresses Would be induced in thecore element as it contracted during cooling below the solidificationtempera-- ture because of its high coefficient of thermal expansionrelative to the core mold material. This would result in failure of thecore element.

The more complex the core configuration desired, the higher thetemperature to which the molten anhydrous calcium chloride should beheated so that it will complete ly fill all of the intricate intersticesor recesses of the core mold cavity before solidifying whereby theresultant fused cast core element will have good dimensional accuracyand surface definition and amount to a truly accurate reproduction ofthe core mold cavity. On the other hand, the molten anhydrous calciumchloride should not be heated to such an extent that undesirable cubicalshrinkage will result as the molten anhydrous calcium chloride coolsdown to the solidification temperature and in practice operatingtechniques can be readily worked out so as to simultaneously meet thesetwo conflicting requirements.

With reference to the drawings, FIGURE 1 illustrates a preferredrepresentative arrangement that may be used in carrying out theteachings of the instant invention during fabrication of the cores ofthe invention and in using them in casting operations. A typicalproduction line is thus shown in FIG. 1 comprised of a series of spacedcore processing stations, diagrammatically represented at 2%, 22, 24,26, 28, 30, 32, 34, 36, 3S, and 40, which schematically portray thevarious operations which can be employed to produce the novel coreelement of the invention, such as elements 14 and 17 of FIGS. 2 and 5,respectively, and to produce an improved hollow product of a desireddesign, such as product 12 in FIGS. 3 and 4. The furnace at the processstation 22 may be of any suitable type, such as an electric or agas-fired furnace, having appropriate temperature control means forcontrolled melting of the salt material of the invention.

After a predetermined amount of the salt from supply station 20 has beendeposited in the furnace 22 and heated to a temperature :sufficient tomaintain it in a molten state and to ensure the removal of all water ofhydration, the fused molten salt is poured or transferred into anappropriately heated core mold of metal such as an electrically heatedcore mold at the process station 24. It is to be understood thatthe coremold may be constructed prom any one of the conventional mold materialssuch as metal, plaster, sand, or the like, and can be of any appropriatedesign having two or more separable sections wherein the contact facesof each section have recesses therein each representing part of theoverall shape of the core element to be made. Although the castingmethod can be of a proper low pressure type, pressure molding may beemployed if desired.

When the salt core has solidified or substantially solidified in thecore mold at station 24, it is promptly removed from the mold. If thecore element is not to be put into immediate usage in the die castingapparatus 30, it must be promptly transferred to station 26 where it isstored in a suitable manner in order to prevent hydration of the saltmaterial. As indicated previously, station 26 may be comprised of apreheating oven maintained at a selected temperature to precludecontamination of the core which will result in hydration.

The die casting apparatus, represented at 30 can be of conventionaldesign. Thus it can, for example, be comprised of the usual frame whichsupports opposed die sections having contacting faces adapted to heforced together and into pressure engagement with each other and with anappropriate core element interposed therebetween. Each one of the diesections has a recess corresponding to part of the overall shape of theproduct to be cast. One or more communicative openings is provided in atleast one of the die sections for injecting the molten metal underpressure into the die cavity defined by the interior faces of the diesections. In die casting aluminum and its alloys, the die sections arenormally preheated to approximately 450 F.

After the cast product is solidified, it is removed from the die casting.apparatus 30 and transferred to station 32. At this station or zone 32the salt cores which have been employed'in the casting are stilldisposed within the casting and are maintained in a condition such asthat shown in FIGURE 4 wherein the salt core is of dimensions which arelarger than the mouth portion of the cavity opening of the casting.

Removal of the disposable salt core element is effected at core removalstation 34 by dissolution of the same in water or any suitable aqueoussolution, preferably heated, and obtained from a suitable sourcerepresented at 38. After the dissolution of the disposable core element,the resultant salt solution is transferred to a separator station 40where the salt is recovered, for example, by evaporation. The recoveredsalt may then be recycled to the salt melting furnace 22 for reuse inadditional core forming operations. The finished cast product with thecore element removed is now transferred to station 36 for furtherdisposition. If desired, the Water from separating station 40 may bereclaimed and reused in core removal station 34.

In FIGURE 2, a disposable core element is shown comprised of large andsmall rectangular solid portions 46 and 48, respectively, interconnectedby a fillet portion 50. The free end of portion 48 includes a terminalsprue section 52 which represents the gate end of the opening in thecore mold. In a core element of an irregular configuration, such aselement 14, the cooling of the different portions 46, 48 and 50 wouldnormally proceed at different rates since the magnitude of the thermalgradient established in cooling a given portion is a function of thesurface area of the mass. Unless controlled, this uneven cooling canresult in undesirable cracking or fracturing of the core element. Inorder to prevent this undesirable cracking or fracturing of the coreelement, it has been found that when a metal core mold is used, the coremold should be heated to an elevated temperature; for example, about 500F. or so depending upon the complexity of the core element configurationand the core element should be removed from the core mold as soon as thesalt of the core element has substantially solidified. Upon removal fromthe core mold the core element should be immediately placed in adepository of the type previously discussed to maintain the core elementin an anhydrous condition.

FIGURES 3 and 4 show a typical cast hollow product which can be made bythe salt core element of the invention. FIGURE 3 represents a hollowcast section 12 having -a cavity 15 of irregular configuration andcomprised of portions 16 and 18 (shown in dotted lines with theconnecting fillet radius not shown). The cavity 15 has been formed bythe injection of metal under high pressures around the core element 1 4'which is shown in place after the casting operation in FIGURE 4. Thecore element of the invention, such .as 14, is able to withstand thehigh compressive forces to which it is subjected in the die castingprocess. Although the core element 14 is of such size and configurationas to prevent physical removal after the casting operation from thecavity 15, its removal may be easily accomplished by dissolving thematerial of the core in water.

Referring now to FIGURE 5, there is shown a further core element 17which has a uniform section 56 of toroidal or doughnut-like shape thatis connected to a cylindrical gate opening section 60 (connecting filletradius not shown) which in turn is connected to a cylindrical spruesection 62. In casting a core element possessed of the complexity ofelement 17, the same conditions as discussed in the case of the coreelement 14 must be observed in order to prevent cracking or fracturingof the element. In the case of core element 17 the toroidal section 56at its inner annular portion would surround an inner portion (not shown)of the core mold, and any substantial difference in the thermalexpansion characteristics between the core mold material and the saltmaterial of the core element would result in cracking or fracturing ofthe core element 17; consequently, the core element 17 should be removedfrom the core mold as soon as solidification is substantially complete.

As an illustrative example of the preparation of a disposable coreelement according to this invention, a commercially pure (9497%) gradeof granulated calcium chloride containing up to other alkali chloridessuch as sodium chloride was employed in the following manner: Themelting furnace was heated to approximately 1600" F. while the core moldwas heated to approximately 500 F. The calcium chloride used had anapproximate melting point of 1422" F. The calcium chloride was heated to.a temperature sufficient to convert and maintain it in the molten stateand to ensure the removal of all water of hydration. The resultantmolten anhydrous calcium chloride was then poured into the preheatedmold and allowed to set until substantially solidified. The fused castbody of anhydrous calcium chloride was then removed from the mold andimmediately transferred to a holding furnace or oven where the coretemperature was maintained at a minimum temperature of 200 C. or 392 F.The core element so produced was found to have good dimensionalaccuracy, surface definition, adequate strength and be an accuratereproduction of the core mold cavity.

The core was used in the sequence of processing steps shown in FIGURE 1to produce the die cast article shown in FIGURE 3. The core element waspreheated to approximately 500 F. and positioned in the die mold. Moltenaluminum alloy at a temperature of approximately 1100 F. was then castinto the die mold around the fused cast core element so as to surroundit. The aluminum alloy used consisted of 3-4% copper, 7.59% silicon,0.7% iron, 0.5% manganese, 0.07% magnesium, 0.5% zinc, 0.3% nickel,others 0.3%, with the balance aluminum. The initial injection pressurefor the molten metal was on the order of 4,000 pounds per square inchand the final injection pressure was on the order of 10,000 pound persquare inch. The molten metal solidified in the die mold and, aftersolidification, the solidified cast metal body surrounding the fusedcast core element was removed from the die,mold. The fused cast coreelement was then removed from within the solidified cast metal body bydissolution in warm water. The die cast article had the desireddimensional accuracy and smooth surfaces requiring virtually nomachining or grinding.

Advantageous embodiments of the invention have been disclosed anddescribed. Although in the foregoing description of the novel coreelements of the invention, the metal employed in the casting exampleshas been aluminum and its alloys, it is to be understood that theinvention is also applicable to the casting of other metals, such asmagnesium, zinc, lead, tin, and their respective alloys. Additionally,it will be obvious that various modifications and alterations may bemade in the invention without departing from the spirit and scopethereof, and it is not to be taken as limited except by the appendedclaims.

What is claimed is:

1. The process of casting metal comprising the steps of (a) heatingcalcium chloride to a tempenature suflicient to convert it to andmaintain it in the molten state and to ensure the removal of all waterof hydration whereby a fused molten body of anhydrous calcium chlorideis obtained,

(b) casting the molten anhydrous calcium chloride into a cavity of amold, the cavity having an overall configuration substantiallycorresponding to the desired shape of the core element to be made,

(c) solidifying the molten anhydrous calcium chloride in the cavity,

(d) removing the fused cast body consisting essentially of anhydrouscalcium chloride from the mold as soon as solidification issubstantially complete,

(e) transferring the fused cast body to a second mold so as to bepositioned therein,

(f) casting molten metal into the second mold around the fused cast bodyso as to surround it,

(g) solidifying the molten metal in the second mold,

(h) removing the solidified cast metal body surrounding the fused castbody from the second mold, and

(i) dissolving the fused cast body from within the solidified cast metalbody.

2. The process of claim 1 wherein the molten metal is injected underpressure into the second mold.

3. In the process of die casting metal around a disposable core elementpositioned in a mold the improvement comprising employing a disposablecore element consisting essentially of a fused cast body of anhydrouscalcium chloride.

References Cited by the Examiner UNITED STATES PATENTS 2,420,851 5/1947Zahn et al. 22-195 2,671,936 3/1954 Sundwick 2.270 3,218,684 11/1965Spink 2 2-196 X References Cited by the Applicant UNITED STATES PATENTS1,523,519 1/1925 Gibons. 1,554,697 9/1925 Alden. 1,603,262 10/ 1926Alden. 3,094,422 6/1963 Reinhold. 3,131,999 5/ 1964 Suzuki et al.

J. SPENCER OVER'HO'LSER, Primary Examiner. E. MAR, Assistant Examiner.

3. IN THE PROCESS OF DIE CASTING METAL AROUND A DISPOSABLE CORE ELEMENTPOSITIONED IN A MOLD THE IMPROVEMENT COMPRISING EMPLOYING A DISPOSABLECORE ELEMENT CONSISTING ESSENTIALLY OF A FUSED CAST BODY OF ANHYDROUSCALCIUM CHLORIDE.